Many head-injured patients require aggressive intravascular volume replacement for the treatment of associated hypovolemic shock, and many isotonic (or hypertonic) asanguinous fluids have been recommended for use in the course of such volume resuscitation. However, very little is known about the specific effects of such fluids on central nervous system (CNS) pathophysiology, and while opinions are plentiful, it is not possible to rationally decide which fluid is best for use in patients with preexistent neurologic damage. These planned experiments will therefore examine (in rabbits) the changes which occur in a large number of important CNS parameters during and following volume replacement with whole blood and with three asanguinous solutions: a) lactated Ringer's, b) hydroxyethyl starch (hetastarch-Hespan), and c) hypertonic lactated Ringer's (480mOsm/1). Monitored CNS variables (in various experiments) will include the electroencephalogram (EEG - with computer assisted analysis), intracranial pressures, global and multi-regional cerebral blood flows and vascular resistances, cerebral metabolic rates for oxygen (CMRO2), glucose (CMRGlucose) and lactate (CMRLactate), cerebral H+ production, cerebrospinal fluid electrolyte, H+ and lactate concentrations, regional brain and spinal cord tissue water and electrolyte contents and neuropathology, along with arterial PO2, electrolytes, acid-base balance, osmotic and oncotic pressures. Such factors will be studied in neurologically normal animals and in animals with a standardized, local traumatic injury, either a) during normotensive, normovolemic hemodilution to a hematocrit of about 20%, or b) during and following resuscitation from a severe hemorrhagic hypovolemic insult (incomplete global cerebral ischemia). The goal of these studies is to evaluate the CNS impact of these different fluids (as comapred to blood), and to determine whether their use for volume replacement/resuscitation is potentially either beneficial or detrimental. The results will have direct applicability to the acute fluid management of patients with neurologic and multi-system trauma and hypovolemic shock. In addition, they may have relevance to the care of patients with other neurosurgical disorders and/or cerebrovascular disease.